The surfaces of optical glass lenses are preferred to be aspheric in recent years, simplifying the composition of a lens in an optical apparatus and reducing its weight. One method of manufacturing aspheric lenses is to grind and polish the glass after fabricating it from a glass block, rod, plate or the like. In a further method, melted glass is pressed with a mold beforehand to form a glass similar to the shape of the desired lens, and the molded glass is then ground and polished. Since the above-noted methods do not facilitate mass production of lenses, a method of forming lenses directly by a metallic mold has recently been promising.
The above-noted method of using a metallic mold includes a direct press method, manufacturing high-precision optical glass components by directly pressing melted glass, and a re-heat press method, providing optical glass components by pressing pre-heated glass materials (preformed materials).
In the direct press method, an optical glass component can be manufactured at a significantly low cost. However, difficulties arise in this method, when treating melted glass at high temperature, to precisely mold the glass, and there are technical problems including shortening the life of the pressing mold. Therefore, it has been difficult to apply this method to practical use.
The re-heat press method, on the other hand, can solve the above-noted problems of the direct press method. However, a relatively high cost is involved in this method, in which the procedures of manufacturing glass material are almost the same as the procedures for manufacturing a polished lens. Thus, simplification of the procedures of manufacturing glass material is an important objective of the re-heat press method.
As an effective and simple method of manufacturing a glass material, the method of dropping melted glass on a mold and letting the dropped glass cool has been proposed. Based on the above-noted method, a glass material used for the re-heat press method can be manufactured at a low cost (for instance, Japanese Patent Application No. Sho 61-146721; Japanese Patent Application No. Hei 2-34525, etc.).
One method of dropping melted glass on a mold is to cut off the melted glass, emerging from the outlet of a glass melting furnace, using a cutter. Another method is to drop-separate the melted glass spontaneously by its own weight.
As a conventional example, FIGS. 3(a)-(c) shows a method of forming a glass material by separating melted glass using a cutter. In FIG. 3(a), the arrangement of melted glass 1 emerging from glass outlet 10 of a glass melting furnace and cutter 13, is shown. In FIG. 3(b), melted glass 1 is dropped on mold 11 after separating the glass by cutter 13. FIG. 3(c) shows defect 5 created on glass material 1. The above-noted method is highly likely to create defect 5, which is a shrinkage crack, bubble or the like generated by quenching inside or on the surface of the glass.
As another conventional example, a method of manufacturing a glass material by dropping melted glass on a mold and spontaneously separating it from a glass outlet by its own weight is shown in FIGS. 4(a)-(d). In FIG. 4(a), melted glass 1 is emerging from glass outlet 10 of a glass melting furnace. In FIG. 4(b), melted glass 1 is separated from the outlet by its own weight, and dropped on the mold 11. Figure (c) shows defect 5 of glass material 1.
In dropping melted glass by its own weight, a pulling force between the weight of the dropping glass and the surface tension (force toward the outlet) of the glass is generated. As a result, the diameter at the separation point of the dropping glass becomes small, thus finally separating the glass from the outlet. The glass around stretched area 3, shaped like a thin needle or a minute protrusion, tends to become colder than the rest of the glass in the dropping process.
Because of the shock created by the glass landing on the mold, the glass around stretched area 3, which is colder than the rest of the glass, falls inside the glass on the mold, thereby generating a difference in temperature and a serious defect such as striae or bubbles inside the glass.
If the surface temperature of the whole dropped glass is lower than its softening temperature, the dropped glass will not restore itself to a preferable form due to its surface tension. In other words, protrusive defect 5 remains on the surface of the glass as in FIG. 4(d).
A defect inside or on the surface of the glass material manufactured by the above-noted methods cannot be eliminated even if the material is manufactured by a metallic mold.